The present invention relates generally to lasers, and more specifically the invention pertains to a diagnostic tool for measuring thermal effects in optically-heated laser rods.
Diode lasers have seen increasing use for optically exciting solid-gain media. As pump power increases, so does heat generation in the gain media. The temperature gradient causes an OPD change through both a refractive index change and thermal expansion. Thermal stress and strain is also generated in the material. The strain in turn causes a larger refractive index change, and the stress may cause material damage. The OPD change results in thermal lensing and birefringence. These effects cause concern over the output beam quality and durability of the optically excited laser. Similar problems exist during high power frequency conversion.
Unwanted heat generated in optically-pumped solid-state lasers causes thermal gradients, strain, and thermal expansion which in turn results in a nonuniform optical path difference (OPD) increase across the gain media. This ultimately results in poor beam quality.
The task of analyzing thermally induced optical path changes of optically pumped lasers is alleviated, to some extent, by the systems disclosed in the following U.S. patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 5,050,989 issued to VanTassel et al;
U.S. Pat. No. 4,007,989 issued to Wajda;
U.S. Pat. No. 4,580,162 issued to Mori;
U.S. Pat. No. 4,442,454 issued to Powell;
U.S. Pat. No. 4,435,838 issued to Gourlay;
U.S. Pat. No. 3,955,891 issued to Knight et al.; and
U.S. Pat. No. 3,578,980 issued to Decker.
While the above cited references are instructive, the ongoing task remains to increase the performance of optically-pumped laser rods.
Old methods of fringe pattern analysis were done with pencil, ruler and paper. While this may work for an unchanging fringe pattern, it would be extremely difficult if not impossible for analysis of a dynamic pattern. Other computer codes written to analyze dynamic patterns use a spatial counting technique, in other words they count the fringe shift by comparing it to some fixed location on the image. Problems with this method include spatial noise buildup and it will not work if there are discontinuities in the fringe pattern or fringe walk off at the boundaries.
Problems associated with heat generation in optically-pumped media necessitate a thorough understanding of the temperature and stress distributions in these materials. Optical interferometry provides a non-intrusive, high resolution method for this purpose, by allowing one to measure the OPD change. Once this is known, an integrated temperature and stress distribution can be extracted in the direction of interest. It is then possible to correct for thermal lensing, study birefringence, optimize cooling schemes, and/or develop a warning for crystal damage.
The number of fringes crossing a given position over time is proportional to the OPD increase for that position.
The purpose of this invention is to count the number of bright fringes which pass through a two-dimensional array of pixel locations over a given time. The number of fringes is directly proportional to the optical path change which created the fringe shift and can be used to analyze the cause of the optical path change, for example the temperature gradient in an optically-pumped laser rod.